It is known that the internal resistance of a fuel cell influences the humidity coefficient of an electrolyte membrane inside the fuel cell. In the case where the amount of the moisture contained in the fuel cell is small and the electrolyte membrane is dry, the internal resistance is high and the output voltage of the fuel cell is low. In the case where the amount of the moisture contained in the fuel cell is excessively large, since an electrode of the fuel cell is covered with the moisture, the diffusion of oxygen and hydrogen as reactants is hampered, and the output voltage is low.
In order to operate the fuel cell with high efficiency, it is necessary to optimally manage the amount of the moisture contained in the fuel cell. It is known that the amount of the moisture contained in the fuel cell is correlated with the impedance of the fuel cell. In Japanese Patent Application Publication No. 2004-15992 (JP-A-2004-15992), an impedance of a fuel cell is measured according to an alternating-current impedance method to indirectly grasp a state of moisture contained in the fuel cell. More specifically, JP-A-2004-15992 describes superimposing a sinusoidal signal having an arbitrary frequency on an output signal of the fuel cell and measuring the impedance of the fuel cell in that case.
However, the number of times of sampling performed in measuring a voltage and a current for the measurement of the impedance is limited due to factors such as a CPU load and the like. Therefore, there is a circumstance in which no sufficient ingenuity is found as to the impedance measurement performance of the fuel cell.